Switching converter with smart frequency generator and control method thereof

ABSTRACT

A control method of a switching converter, wherein the switching converter has a main transistor and is configured to provide an output signal. The control method includes: generating a feedback signal indicative of the output signal of the switching converter; generating a clock signal to determine the switching frequency of the main transistor; generating a control signal to control the main transistor based on the clock signal and the feedback signal; and detecting whether the on-time of the main transistor is smaller than a time threshold based on the control signal. If the on-time of the main transistor is smaller than the time threshold, the frequency of the clock signal will be adjusted to regulate the on-time of the main transistor to be equal to the time threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of CN application 201410841384.X,filed on Dec. 30, 2014 and incorporated herein by reference.

TECHNICAL FIELD

The present invention generally relates to electronic circuits, and moreparticularly but not exclusively to switching converters.

BACKGROUND

Peak current control method is widely used in switching converters. Inpeak current control, a reference voltage and a feedback signal whichindicates the output voltage of the switching converter are sent into anerror amplifier to generate a compensation signal. The main transistorof the switching converter will become on once a clock signal comes, andbecome off when a current sensing signal indicative of the currentflowing through the main transistor hits the compensation signal. Owingto the inherent delay of the control circuit, there exists a minimumon-time, wherein the main transistor can turn off only after its on-timereaches this minimum on time.

If the input voltage of the switching converter keeps increasing, theon-time of the main transistor will decrease until it reaches theminimum on-time. Hereafter, if the input voltage continues rising, theoutput voltage of the switching voltage will go up and a large ripplewould arise accordingly.

SUMMARY

To solve the problem mentioned above, the present invention involves atime threshold which is larger than the minimum on-time. If the on-timeof the main transistor becomes smaller than the time threshold, thefrequency of the clock signal would be adjusted to regulate the on-timeof the main transistor to be equal to the time threshold. By doing so,the on-time of the main transistor would not fall to reach the minimumon-time anymore, thus the voltage ripple potentially caused by theminimum on-time can be fundamentally avoided.

Embodiments of the present invention are directed to a switchingconverter, comprising: a switching circuit having a main transistor,wherein the switching circuit is configured to convert an input voltageinto an output voltage; a feedback circuit coupled to the switchingcircuit, wherein the feedback circuit is configured to generate afeedback signal indicative of the output voltage; a current sensingcircuit configured to sense the current flowing through the maintransistor and generate a current sensing signal; a clock generatorconfigured to generate a clock signal; an error amplifying circuitcoupled to the feedback circuit, wherein based on the difference betweena first reference voltage and the feedback signal, the error amplifyingcircuit generates a compensation signal; a comparing circuit coupled tothe current sensing circuit and the error amplifying circuit, whereinthe comparing circuit compares the current sensing signal with thecompensation signal and generates a reset signal; and a control circuitcoupled to the clock generator and the comparing circuit, wherein basedon the clock signal and the reset signal, the control circuit generatesa control signal to control the main transistor. The clock generator iscoupled to the control circuit to receive the control signal and detectwhether the on-time of the main transistor is smaller than a timethreshold based on the control signal. If the on-time of the maintransistor is smaller than the time threshold, the clock generator willadjust the frequency of the clock signal to regulate the on-time of themain transistor to be equal to the time threshold.

Embodiments of the present invention are also directed to a controllerused in a switching converter, wherein the switching converter has amain transistor and is configured to provide an output signal. Thecontroller comprises: a clock generator configured to generate a clocksignal to determine the switching frequency of the main transistor; anda control circuit coupled to the clock generator, wherein based on theclock signal and a feedback signal indicative of the output signal ofthe switching converter, the control circuit generates a control signalto control the main transistor. If the on-time of the main transistor issmaller than a time threshold, the clock generator will adjust thefrequency of the clock signal to regulate the on-time of the maintransistor to be equal to the time threshold.

Embodiments of the present invention are further directed to a controlmethod of a switching converter, wherein the switching converter has amain transistor and is configured to provide an output signal. Thecontrol method comprises: generating a feedback signal indicative of theoutput signal of the switching converter; generating a clock signal todetermine the switching frequency of the main transistor; generating acontrol signal to control the main transistor based on the clock signaland the feedback signal; detecting whether the on-time of the maintransistor is smaller than a time threshold based on the control signal;and if the on-time of the main transistor is smaller than the timethreshold, adjusting the frequency of the clock signal to regulate theon-time of the main transistor to be equal to the time threshold.

BRIEF DESCRIPTION OF THE DRAWING

The present invention can be further understood with reference to thefollowing detailed description and the appended drawings, wherein likeelements are provided with like reference numerals.

FIG. 1 is a block diagram of a switching converter 100 in accordancewith an embodiment of the present invention.

FIG. 2A schematically illustrates a clock generator 102A in accordancewith an embodiment of the present invention.

FIG. 2B illustrates working waveforms of the clock generator 102A shownin FIG. 2A in accordance with an embodiment of the present invention.

FIG. 3 schematically illustrates a clock generator 102B in accordancewith an embodiment of the present invention.

FIG. 4 schematically illustrates a clock generator 102C in accordancewith an embodiment of the present invention.

FIG. 5 schematically illustrates a clock generator 102D in accordancewith an embodiment of the present invention.

FIG. 6 schematically illustrates a clock generator 102E in accordancewith an embodiment of the present invention.

FIG. 7 schematically illustrates a switching converter 100A inaccordance with an embodiment of the present invention.

FIG. 8 is a flow chart of a control method for switching converters inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail so as not to unnecessarily obscure aspects of the presentinvention.

To solve the problem mentioned in the background, the present inventionsets a time threshold (e.g. 80 ns) larger than the minimum on-time (e.g.50 ns), and adjusts the frequency of the clock signal when the on-timeof the main transistor becomes smaller than the time threshold, so as toregulate the on-time of the main transistor to be equal to the timethreshold. Therefore, the on-time of the main transistor would not fallto reach the minimum on-time, and the voltage ripple caused by theminimum on-time can be fundamentally eliminated.

FIG. 1 is a block diagram of a switching converter 100 in accordancewith an embodiment of the present invention. The switching converter 100includes a switching circuit 101, a clock generator 102, a controlcircuit 103, a feedback circuit 104, a comparing circuit 105, an erroramplifying circuit 106 and a current sensing circuit 107. The switchingcircuit 101 has a main transistor, and is configured to convert an inputvoltage Vin into an output voltage Vout. The switching circuit 101 canbe configured in any suitable topologies, such as BUCK, BOOST,BUCK-BOOST, FLYBACK, etc. The feedback circuit 104 is coupled to theswitching circuit 101. It senses an output signal of the switchingcircuit 101 and generates a feedback signal FB accordingly. The outputsignal can be the output voltage Vout shown in FIG. 1, or alternatively,the output current or output power of the switching circuit 101.

The error amplifying circuit 106 is coupled to the feedback circuit 104,wherein based on the difference between a reference voltage Vref1 andthe feedback signal FB, the error amplifying circuit 106 generates acompensation signal COMP. The current sensing circuit 107 is configuredto sense the current flowing through the main transistor and generate acurrent sensing signal ISENSE. The comparing circuit 105 is coupled tothe current sensing circuit 107 and the error amplifying circuit 106,wherein the comparing circuit 105 compares the current sensing signalISENSE with the compensation signal COMP and generates a reset signalRST. The clock generator 102 is configured to generate a clock signalCLK. The control circuit 103 is coupled to the clock generator 102 andthe comparing circuit 105, wherein based on the clock signal CLK and thereset signal RST, the control circuit 103 generates a control signalCTRL to control the main transistor in the switching circuit 101.

As can be seen from FIG. 1, the clock generator 102 is coupled to thecontrol circuit 103 to receive the control signal CTRL. It detectswhether the on-time Ton of the main transistor is smaller than a timethreshold Tth based on the control signal CTRL. The frequency of theclock signal CLK is normally constant. But if the on-time Ton is smallerthan the time threshold Tth, the clock generator 102 will adjust thefrequency of the clock signal CLK to regulate the on-time Ton to beequal to the time threshold Tth. Generally, the time threshold Tth isconfigured to be larger than the minimum on-time of the main transistor.

It is well-known that, under the same load condition, an increase of theclock frequency would cause the on-time Ton to decrease, and vice versa.When the on-time Ton reduces to be smaller than the time threshold Tthdue to an increase of the input voltage Vin, the on-time Ton would beregulated to be equal to the time threshold Tth. The regulation of theenergy provided to the load is now realized through adjusting the clockfrequency, and the voltage ripple potentially caused by the minimumon-time is fundamentally eliminated.

In some embodiments, to prevent misjudgment, the clock generator 102adjusts the frequency of the clock signal CLK only when the on-time Tonof the main transistor is smaller than the time threshold Tth in aplurality of successive switching cycles.

FIG. 2A schematically illustrates a clock generator 102A in accordancewith an embodiment of the present invention. The clock generator 102Acomprises a first current control circuit 221A, a controllable currentsource 222A, a current mirror 223A, a frequency setting circuit 224A, acapacitor C1, a transistor T4, a comparator COM1 and a one-shot circuit225A. The first current control circuit 221A has an input terminal andan output terminal, wherein the input terminal is coupled to the controlcircuit to receive the control signal CTRL, and wherein based on thecontrol signal CTRL, the first current control circuit 221A detectswhether the on-time Ton of the main transistor is smaller than the timethreshold Tth, and generates a first current control signal CCS1 at theoutput terminal. The controllable current source 222A has a firstterminal, a second terminal and a control terminal, wherein the firstterminal is coupled to a power supply voltage Vcc, the control terminalis coupled to the output terminal of the first current control circuit221A to receive the first current control signal CCS1, the secondterminal is configured to provide a current I1. The frequency settingcircuit 224A is configured to provide a setting current Iset. In theembodiment shown in FIG. 2A, the frequency setting circuit 224A includesa transistor T3, a resistor R1 and an operational amplifier AMP1. Peopleskilled in the art can recognize, however, that the frequency settingcircuit 224A may be configured in other suitable structures, such as acurrent source controlled by an external clock signal.

As can be seen from FIG. 2A, the current mirror 223A includestransistors T1 and T2. It has a power supply terminal, a first terminaland a second terminal, wherein the power supply terminal is coupled tothe power supply voltage Vcc, the first terminal is coupled to thefrequency setting circuit 224A and the second terminal of thecontrollable current source 222A. The current Ichg provided at thesecond terminal of the current mirror 223A can be expressed as:

I _(chg) =I _(set) −I ₁   (1.1)

The capacitor C1 has a first terminal and a second terminal, wherein thefirst terminal is coupled to the second terminal of the current mirror223A, the second terminal is coupled to a reference ground. Thetransistor T4 has a first terminal, a second terminal and a controlterminal, wherein the first terminal is coupled to the first terminal ofthe capacitor C1, the second terminal is coupled to the referenceground. The comparator COM1 has a first input terminal, a second inputterminal and an output terminal, wherein the first input terminal iscoupled to the first terminal of the capacitor C1, the second inputterminal is configured to receive a threshold voltage Vth. Thecomparator COM1 compares the voltage Vc1 across the capacitor C1 withthe threshold voltage Vth and generates a comparison signal CMPO at theoutput terminal. The one-shot circuit 225A has an input terminal and anoutput terminal, wherein the input terminal is coupled to the outputterminal of the comparator COM1, the output terminal is coupled to thecontrol terminal of the transistor T4 and is configured to provide theclock signal CLK.

FIG. 2B illustrates working waveforms of the clock generator 102A inaccordance with an embodiment of the present invention. As shown in thefigure, when the clock signal CLK is logical low, the transistor T4 isoff. The capacitor C1 is charged by the current Ichg and the voltage Vc1across the capacitor C1 gradually increases. Once the voltage Vc1increases to reach the threshold voltage Vth, the comparison signal CMPOchanges from logical low into logical high. The one-shot circuit 225A istriggered to generate a pulse at the clock signal CLK, so the transistorT4 is turned on for a time period to discharge the capacitor C1.

The frequency fclk of the clock signal CLK can be expressed as:

$\begin{matrix}{f_{clk} = {\frac{1}{t_{chg} + t_{pulse}} = \frac{1}{{C_{1} \times \frac{V_{th}}{I_{chg}}} + t_{pulse}}}} & (1.2)\end{matrix}$

Wherein tchg represents the charge time of the capacitor C1, and tpulserepresents the pulse width of the clock signal CLK.

Combing the equations (1.2) and (1.1), we can get:

$\begin{matrix}{f_{clk} = \frac{1}{{C_{1} \times \frac{V_{th}}{I_{set} - I_{1}}} + I_{pulse}}} & (1.3)\end{matrix}$

It is apparent from the equation (1.3) that the clock frequency fclkwould be affected by the current I1 which is provided by thecontrollable current source 222A. The clock frequency fclk decreaseswhen the current I1 increases, and vice versa.

When the on-time Ton of the main transistor is smaller than the timethreshold Tth, the current I1 varies under the control of the firstcurrent control signal CCS1, so as to regulate the on-time Ton to beequal to the time threshold Tth. On the other side, when the on-time Tonis larger than the time threshold Tth, the current I1 is zero and theclock frequency fclk is a constant value determined by the settingcurrent Iset.

FIG. 3 schematically illustrates a clock generator 102B in accordancewith an embodiment of the present invention. As shown in FIG. 3, thefirst current control circuit 221 B comprises a one-shot circuit 3211, aflip-flop FF1, a logic circuit 3212, current source IS1, IS2,transistors T5, T6 and a capacitor C2. The one-shot circuit 3211 has aninput terminal and an output terminal, wherein the input terminal iscoupled to the control circuit to receive the control signal CTRL. Theflip-flop FF1 has a clock input terminal, a data input terminal and anoutput terminal, wherein the clock input terminal is coupled to theoutput terminal of the one-shot circuit 3211, the data input terminal iscoupled to the control circuit to receive the control signal CTRL, theoutput terminal is configured to provide an on-time detection signalDEC. The logic circuit 3212 has an input terminal, a first outputterminal and a second output terminal, wherein the input terminal iscoupled to the output terminal of the flip-flop FF1 to receive theon-time detection signal DEC, and wherein based on the on-time detectionsignal DEC, the logic circuit 3212 generates logic signals LOG1 and LOG2respectively at the first output terminal and the second outputterminal. The current source IS1 has a first terminal and a secondterminal, wherein the first terminal is coupled to the power supplyvoltage Vcc. The transistor T5 has a first terminal, a second terminaland a control terminal, wherein the first terminal is coupled to thesecond terminal of the current source IS1, and the control terminal iscoupled to the first output terminal of the logic circuit 3212 toreceive the logic signal LOG1. The transistor T6 has a first terminal, asecond terminal and a control terminal, wherein the first terminal iscoupled to the second terminal of the transistor T5, and the controlterminal is coupled to the second output terminal of the logic circuit3212 to receive the logic signal LOG2. The current source IS2 has afirst terminal and a second terminal, wherein the first terminal iscoupled to the second terminal of the transistor T6, the second terminalis coupled to the reference ground. The capacitor C2 has a firstterminal and a second terminal, wherein the first terminal is coupled tothe second terminal of the transistor T5 and the first terminal of thetransistor T6, and is configured to provide the first current controlsignal CCS1, the second terminal is coupled to the reference ground.

The controllable current source 222B includes a current mirror 3213, atransistor T7, a resistor R2 and a current source IS3. The transistor T7has a first terminal, a second terminal and a control terminal, whereinthe control terminal is coupled to the first current control circuit 221B to receive the first current control signal CCS1. The resistor R2 hasa first terminal and a second terminal, wherein the first terminal iscoupled to the second terminal of the transistor T7. The current sourceIS3 has a first terminal and a second terminal, wherein the firstterminal is coupled to the second terminal of the resistor R2, thesecond terminal is coupled to the reference ground. The current mirror3212 includes transistors T8 and T9. It has a power supply terminal, afirst terminal and a second terminal, wherein the power supply terminalis coupled to the power supply voltage Vcc, the first terminal iscoupled to the first terminal of the transistor T7, the second terminalis configured to provide the current I1.

When the control signal CTRL changes from logical low into logical high,the one-shot circuit 3211 is triggered to generate a pulse signal whichhas a pulse width equal to the time threshold Tth. At the falling edgeof the pulse signal, the flip-flop FF1 is triggered to provide thesignal at its data input terminal to the output terminal. If the on-timeTon of the main transistor is smaller than the time threshold, theon-time detection signal DEC output by the flip-flop FF1 would belogical low. Under the control of the logic circuit 3212, the transistorT6 turns off and the transistor T5 turns on for a time period to let thecurrent source IS1 charge the capacitor C2. The first current controlsignal CCS1 which is equal to the voltage across the capacitor C2 goesup. When the first current control signal CCS1 becomes larger than thethreshold voltage of the transistor T7, the current I1 generated by thecurrent mirror 3123 would be larger than zero and vary along with thefirst current control signal CCS1. Specifically speaking, the maximumvalue of the current I1 is determined by the current source IS3.

If the on-time Ton of the main transistor is larger than the timethreshold, the on-time detection signal DEC output by the flip-flop FF1would be logical high. Under the control of the logic circuit 3212, thetransistor T5 turns off and the transistor T6 turns on for a time periodto let the current source IS2 discharge the capacitor C2. The current I1generated by the current mirror 3123 as well as the first currentcontrol signal CCS1 goes down. When the first current control signalCCS1 becomes smaller than the threshold voltage of the transistor T7,the current I1 would be zero and the clock frequency fclk would resumeto the constant value mentioned before.

In one embodiment, the first current control circuit 221 B furthercomprises a fast startup circuit including a current source IS4,transistors T10˜T12 and a multi detection circuit 3214. The currentsource IS4 has a first terminal and a second terminal, wherein the firstterminal is coupled to the power supply voltage Vcc. The transistor T10has a first terminal, a second terminal and a control terminal, whereinthe first terminal is coupled to the power supply voltage Vcc, thesecond terminal is coupled to the first terminal of the capacitor C2,the control terminal is coupled to the second terminal of the currentsource IS4. The transistor T11 has a first terminal, a second terminaland a control terminal, wherein the first terminal is coupled to thecontrol terminal of the transistor T10, the second terminal is coupledto the reference ground, the control terminal is coupled to the firstterminal of the capacitor C2. The multi detection circuit 3214 iscoupled to the output terminal of the flip-flop FF1 to receive theon-time detection signal DEC, wherein based on the on-time detectionsignal DEC, the multi detection circuit 3214 determines whether theon-time Ton of the main transistor is smaller than the time thresholdTth in a plurality of successive switching cycles, and generates a multidetection signal MTD. The transistor T12 has a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the control terminal of the transistor T10, the second terminal iscoupled to the reference ground, and the control terminal is coupled tothe multi detection circuit 3214 to receive the multi detection signalMTD.

In normal operation, the transistor T12 is on and the transistor T10 isoff. The fast startup circuit does not work. When the multi detectioncircuit 3214 detects the on-time Ton of the main transistor is smallerthan the time threshold in a plurality of successive switching cycles,the transistor T12 turns off and the fast startup circuit starts towork. The capacitor C2 is charged by the power supply voltage Vccthrough the transistor T10 until the current flowing through thetransistor T11 becomes equal to the current provided by the currentsource IS4.

FIG. 4 schematically illustrates a clock generator 102C in accordancewith an embodiment of the present invention, wherein the first currentcontrol circuit 221C comprises transistors T13˜T15, a current source IS5and a capacitor C3. The transistor T13 has a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the power supply voltage Vcc, the control terminal is coupled to thecontrol terminals of the transistors T1 and T2. The transistor T14 has afirst terminal, a second terminal and a control terminal, wherein thefirst terminal is coupled to the second terminal of the transistor T13.The transistor T15 has a first terminal, a second terminal and a controlterminal, wherein the first terminal is coupled to the second terminalof the transistor T14, the control terminal is coupled to the controlcircuit to receive the control signal CTRL. The current source IS5 has afirst terminal and a second terminal, wherein the first terminal iscoupled to the second terminal of the transistor T15, the secondterminal is coupled to the reference ground. The capacitor C3 has afirst terminal and a second terminal, wherein the first terminal iscoupled to the second terminal of the transistor T14 and the firstterminal of the transistor T15, and is configured to provide the firstcurrent control signal CCS1, the second terminal is coupled to thereference ground.

The transistor T14 is maintained on in current continuous mode. Thetransistor T15 turns on when the main transistor is on and turns offwhen the main transistor is off. Based on the configuration shown inFIG. 4, the time threshold Tth can be expressed as:

$\begin{matrix}{T_{th} = \frac{I_{chg}}{I_{s\; 5} \times f_{clk}}} & (1.4)\end{matrix}$

When the on-time Ton of the main transistor is larger than the timethreshold Tth as shown in the equation (1.4), the first current controlsignal CCS1 which is equal to the voltage across the capacitor C3 wouldbe lower than the threshold voltage of the transistor T7. Thus thetransistor T7 turns off and the current I1 is zero. When the on-time Tonis smaller than the time threshold Tth, the first current control signalCCS1 would increase to be higher than the threshold voltage of thetransistor T7. Consequently, the transistor T7 turns on and the currentI1 becomes larger than zero.

FIG. 5 schematically illustrates a clock generator 102D in accordancewith an embodiment of the present invention. Compared with the clockgenerator 102A shown in FIG. 2, the clock generator 103D of FIG. 5further comprises a second current control circuit 525 and acontrollable current source 526. The second current control circuit 525is configured to detect whether the switching converter works under alight load condition and generate a second current control signal CCS2.The controllable current source 525 has a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the power supply voltage Vcc, the second terminal is coupled to thefirst terminal of the current mirror 223A to provide a current I2, thecontrol terminal is coupled to the second current control circuit 525 toreceive the second current control signal CCS2. Therefore, the currentIchg generated by the current mirror 223A can be expressed as:

I _(chg) =I _(set) −I ₁ −I ₂   (1.5)

If the second current control circuit 525 detects that the switchingconverter does not work under the light load condition, the current I2output by the controllable current source 526 will be zero. Otherwise,if the second current control circuit 525 detects that the switchingconverter works under the light load condition, which means the outputcurrent or output power of the switching converter is lower than apredetermined value, the current I2 output by the controllable currentsource 526 will be larger than zero. The clock frequency fclk as well asthe current Ichg will decrease, which definitely lowers the switchingloss of the switching converter and improves the light load efficiency.

The function of the second current control circuit 525 and thecontrollable current source 526 can be realized by a circuit 627 shownin FIG. 6. The circuit 627 includes a controllable current source 6271,transistors T16˜T19 and a current source IS6. The controllable currentsource 6271 has a first terminal, a second terminal and a controlterminal, wherein the first terminal is coupled to the power supplyvoltage Vcc, the control terminal is coupled to the error amplifyingcircuit to receive the compensation signal COMP. The controllablecurrent source 6271 provides a current Icomp determined by thecompensation signal COMP. The transistor T16 has a first terminal, asecond terminal and a control terminal, wherein the first terminal iscoupled to the power supply voltage Vcc, the second terminal is coupledto the reference ground. The transistor T17 has a first terminal, asecond terminal and a control terminal, wherein the first terminal andthe control terminal are coupled to the second terminal of thecontrollable current source 6271. The transistor T18 has a firstterminal, a second terminal and a control terminal, wherein the firstterminal is coupled to the first terminal of the current mirror 223A,the second terminal is coupled to the frequency setting circuit 224A,the control terminal is coupled to the control terminal of thetransistor T17. The transistor T19 has a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the power supply voltage Vcc, the second terminal is coupled to thefrequency setting circuit 224A, the control terminal is coupled to thecontrol terminal of the transistor T16. The current source IS6 has afirst terminal and a second terminal, wherein the first terminal iscoupled to the second terminals of the transistors T16 and T17, thesecond terminal is coupled to the reference ground.

Based on the configuration described above, it can be derived that:

I ₁₆ +I ₁₇ =I _(s6)   (1.6)

I ₁₈ +I ₁₉ =I _(set)   (1.7)

I ₁₆ ×I ₁₈ =I ₁₇ ×I ₁₉   (1.8)

I₁₇=I_(comp)   (1.9)

I _(chg) =I ₁₈ −I ₁   (1.10)

Wherein I16˜I19 respectively represent the current flowing through thetransistors T16˜T19.

When the switching converter does not works under the light loadcondition, the current Icomp output by the controllable current source6271 is larger than the current provided by the current source IS6. Thecurrent I16 flowing through the transistor T16 and the current I19flowing through the transistor T19 are both zero. Then just as shown inthe equation (1.1), the current Ichg generated by the current mirror223A would not be affected by the circuit 627.

When the switching converter works under light load condition, thecurrent Icomp becomes smaller than the current provided by the currentsource IS6. The current Ichg generated by the current mirror 223A can beexpressed as:

$\begin{matrix}{I_{chg} = {\frac{I_{comp} \times I_{set}}{I_{s\; 6}} - I_{1}}} & (1.11)\end{matrix}$

It can be seen from the equation (1.11) that, the lighter the load, thesmaller the compensation signal COMP, thus the lower the current Ichgand the clock frequency fclk.

FIG. 7 schematically illustrates a switching converter 100A inaccordance with an embodiment of the present invention. The switchingcircuit 101A is a synchronous BUCK circuit consisting of an inputcapacitor Cin, transistors S1, S2, an inductor L and an output capacitorCout, connected as shown in FIG. 7. The feedback circuit 104A comprisesa resistor divider composed of resistors R3 and R4. The error amplifyingcircuit 106A contains an error amplifier AMP2. The comparing circuit105A involves a comparator COM2 which compares a sum of the currentsensing signal ISENSE and a ramp signal RAMP with the compensationsignal COMP and generates the reset signal RST. The control circuit 103Aincludes a flip-flop FF2 having a set terminal, a reset terminal and anoutput terminal, wherein the set terminal is coupled to the clockgenerator 102 to receive the clock signal CLK, the reset terminal iscoupled to the comparing circuit 105A to receive the reset signal RST,the output terminal is configured to provide the control signal CTRLwhich controls the transistors Si and S2 through a driving circuit 708.

Although the embodiments shown in FIG. 1 and FIG. 7 both utilize thepeak current control method, people of ordinary skill in the art canrecognize that the error amplifying circuit, current sensing circuit andcomparing circuit are not necessary, and the present invention can beapplied to other suitable control schemes, e.g. single loop PWM control.Furthermore, in the embodiments described above, the output current ofthe current mirror is equal to the input current. But it can beunderstood that this does not intend to limit the present invention, andthe output current of the current mirror can also be proportional to theinput current.

FIG. 8 is a flow chart of a control method for switching converters inaccordance with an embodiment of the present invention. It comprisessteps S801˜S805.

At step S801, a feedback signal indicative of the output signal of theswitching converter is generated.

At step S802, a clock signal configured to determine the switchingfrequency of the main transistor is generated.

At step S803, a control signal is generated to control the maintransistor based on the clock signal and the feedback signal.

At step S804, whether the on-time of the main transistor Ton is smallerthan a time threshold Tth is detected based on the control signal. Ifthe on-time Ton is smaller than the time threshold Tth, proceed to stepS805, otherwise, keep detecting.

At step S805, the frequency of the clock signal is adjusted to regulatethe on-time Ton to be equal to the time threshold Tth.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described. It should beunderstood, of course, the foregoing disclosure relates only to apreferred embodiment (or embodiments) of the invention and that numerousmodifications may be made therein without departing from the spirit andthe scope of the invention as set forth in the appended claims. Variousmodifications are contemplated and they obviously will be resorted to bythose skilled in the art without departing from the spirit and the scopeof the invention as hereinafter defined by the appended claims as only apreferred embodiment(s) thereof has been disclosed.

What is claimed is:
 1. A switching converter, comprising: a switchingcircuit having a main transistor, wherein the switching circuit isconfigured to convert an input voltage into an output voltage; afeedback circuit coupled to the switching circuit, wherein the feedbackcircuit is configured to generate a feedback signal indicative of theoutput voltage; a current sensing circuit configured to sense thecurrent flowing through the main transistor and generate a currentsensing signal; a clock generator configured to generate a clock signal;an error amplifying circuit coupled to the feedback circuit, whereinbased on the difference between a first reference voltage and thefeedback signal, the error amplifying circuit generates a compensationsignal; a comparing circuit coupled to the current sensing circuit andthe error amplifying circuit, wherein the comparing circuit compares thecurrent sensing signal with the compensation signal and generates areset signal; and a control circuit coupled to the clock generator andthe comparing circuit, wherein based on the clock signal and the resetsignal, the control circuit generates a control signal to control themain transistor; wherein the clock generator is coupled to the controlcircuit to receive the control signal and detect whether the on-time ofthe main transistor is smaller than a time threshold based on thecontrol signal, and wherein if the on-time of the main transistor issmaller than the time threshold, the clock generator will adjust thefrequency of the clock signal to regulate the on-time of the maintransistor to be equal to the time threshold.
 2. The switching converterof claim 1, wherein the clock generator adjusts the frequency of theclock signal only when the on-time of the main transistor is smallerthan the time threshold in a plurality of successive switching cycles.3. The switching converter of claim 1, wherein the clock generatorcomprises: a first current control circuit having an input terminal andan output terminal, wherein the input terminal is coupled to the controlcircuit to receive the control signal, and wherein based on the controlsignal, the first current control circuit detects whether the on-time ofthe main transistor is smaller than the time threshold, and generates afirst current control signal at the output terminal; a firstcontrollable current source having a first terminal, a second terminaland a control terminal, wherein the first terminal is coupled to a powersupply voltage, the control terminal is coupled to the output terminalof the first current control circuit; a frequency setting circuitconfigured to provide a setting current; a first current mirrorincluding a first transistor and a second transistor, wherein the firstcurrent mirror has a power supply terminal, a first terminal and asecond terminal, and wherein the power supply terminal is coupled to thepower supply voltage, the first terminal is coupled to the frequencysetting circuit and the second terminal of the first controllablecurrent source; a first capacitor having a first terminal and a secondterminal, wherein the first terminal is coupled to the second terminalof the first current mirror, the second terminal is coupled to areference ground; a fourth transistor having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the first terminal of the first capacitor, the second terminal iscoupled to the reference ground; a comparator having a first inputterminal, a second input terminal and an output terminal, wherein thefirst input terminal is coupled to the first terminal of the firstcapacitor, the second input terminal is configured to receive athreshold voltage; and a first one-shot circuit having an input terminaland an output terminal, wherein the input terminal is coupled to theoutput terminal of the comparator, the output terminal is coupled to thecontrol terminal of the fourth transistor and is configured to providethe clock signal.
 4. The switching converter of claim 3, wherein thefrequency setting circuit comprises: a third transistor having a firstterminal, a second terminal and a control terminal, wherein the firstterminal is coupled to the second terminal of the first controllablecurrent source and the first terminal of the first current mirror; anoperational amplifier having a first input terminal, a second inputterminal and an output terminal, wherein the first input terminal isconfigured to receive a second reference voltage, the second inputterminal is coupled to the second terminal of the third transistor, theoutput terminal is coupled to the control terminal of the thirdtransistor; and a first resistor having a first terminal and a secondterminal, wherein the first terminal is coupled to the second terminalof the third transistor, the second terminal is coupled to the referenceground.
 5. The switching converter of claim 3, wherein the first currentcontrol circuit comprises: a second one-shot circuit having an inputterminal and an output terminal, wherein the input terminal is coupledto the control circuit to receive the control signal; a flip-flop havinga clock input terminal, a data input terminal and an output terminal,wherein the clock input terminal is coupled to the output terminal ofthe second one-shot circuit, the data input terminal is coupled to thecontrol circuit to receive the control signal, the output terminal isconfigured to provide an on-time detection signal; a logic circuithaving an input terminal, a first output terminal and a second outputterminal, wherein the input terminal is coupled to the output terminalof the flip-flop, and wherein based on the on-time detection signal, thelogic circuit generates a first logic signal and a second logic signalrespectively at the first output terminal and the second outputterminal; a first current source having a first terminal and a secondterminal, wherein the first terminal is coupled to the power supplyvoltage; a fifth transistor having a first terminal, a second terminaland a control terminal, wherein the first terminal is coupled to thesecond terminal of the first current source, the control terminal iscoupled to the first output terminal of the logic circuit to receive thefirst logic signal; a sixth transistor having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the second terminal of the fifth transistor, the control terminal iscoupled to the second output terminal of the logic circuit to receivethe second logic signal; a second current source having a first terminaland a second terminal, wherein the first terminal is coupled to thesecond terminal of the sixth transistor, the second terminal is coupledto the reference ground; and a second capacitor having a first terminaland a second terminal, wherein the first terminal is coupled to thesecond terminal of the fifth transistor and the first terminal of thesixth transistor, and is configured to provide the first current controlsignal, the second terminal is coupled to the reference ground.
 6. Theswitching converter of claim 3, wherein the first controllable currentsource comprises: a seventh transistor having a first terminal, a secondterminal and a control terminal, wherein the control terminal is coupledto the first current control circuit to receive the first currentcontrol signal; a second resistor having a first terminal and a secondterminal, wherein the first terminal is coupled to the second terminalof the seventh transistor; a third current source having a firstterminal and a second terminal, wherein the first terminal is coupled tothe second terminal of the second resistor, the second terminal iscoupled to the reference ground; and a second current mirror includingan eighth transistor and a ninth transistor, wherein the second currentmirror has a power supply terminal, a first terminal and a secondterminal, wherein the power supply terminal is coupled to the powersupply voltage, the first terminal is coupled to the first terminal ofthe seventh transistor, the second terminal is coupled to the firstterminal of the first current mirror.
 7. The switching converter ofclaim 5, wherein the first current control circuit further comprises: afourth current source having a first terminal and a second terminal,wherein the first terminal is coupled to the power supply voltage; atenth transistor having a first terminal, a second terminal and acontrol terminal, wherein the first terminal is coupled to the powersupply voltage, the second terminal is coupled to the first terminal ofthe second capacitor, the control terminal is coupled to the secondterminal of the fourth current source; an eleventh transistor having afirst terminal, a second terminal and a control terminal, wherein thefirst terminal is coupled to the control terminal of the tenthtransistor, the second terminal is coupled to the reference ground, thecontrol terminal is coupled to the first terminal of the secondcapacitor; a multi detection circuit coupled to the output terminal ofthe flip-flop, wherein based on the on-time detection signal, the multidetection circuit determines whether the on-time of the main transistoris smaller than the time threshold in a plurality of successiveswitching cycles, and generates a multi detection signal; and a twelfthtransistor having a first terminal, a second terminal and a controlterminal, wherein the first terminal is coupled to the control terminalof the tenth transistor, the second terminal is coupled to the referenceground, and the control terminal is coupled to the multi detectioncircuit to receive the multi detection signal.
 8. The switchingconverter of claim 3, wherein the first current control circuitcomprises: a thirteenth transistor having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the power supply voltage, the control terminal is coupled to thecontrol terminals of the first transistor and the second transistor; afourteenth transistor having a first terminal, a second terminal and acontrol terminal, wherein the first terminal is coupled to the secondterminal of the thirteenth transistor; a fifteenth transistor having afirst terminal, a second terminal and a control terminal, wherein thefirst terminal is coupled to the second terminal of the fourteenthtransistor, the control terminal is coupled to the control circuit toreceive the control signal; a fifth current source having a firstterminal and a second terminal, wherein the first terminal is coupled tothe second terminal of the fifteenth transistor, the second terminal iscoupled to the reference ground; and a third capacitor having a firstterminal and a second terminal, wherein the first terminal is coupled tothe second terminal of the fourteenth transistor and the first terminalof the fifteenth transistor, and is configured to provide the firstcurrent control signal, the second terminal is coupled to the referenceground.
 9. The switching converter of claim 3, wherein the clockgenerator further comprises: a second current control circuit configuredto detect whether the switching converter works under a light loadcondition and generate a second current control signal; and a secondcontrollable current source having a first terminal, a second terminaland a control terminal, wherein the first terminal is coupled to thepower supply voltage, the second terminal is coupled to the firstterminal of the first current mirror, the control terminal is coupled tothe second current control circuit to receive the second current controlsignal.
 10. The switching converter of claim 3, wherein the clockgenerator further comprises: a third controllable current source havinga first terminal, a second terminal and a control terminal, wherein thefirst terminal is coupled to the power supply voltage, the controlterminal is coupled to the error amplifying circuit to receive thecompensation signal; a sixteenth transistor having a first terminal, asecond terminal and a control terminal, wherein the first terminal iscoupled to the power supply voltage, the second terminal is coupled tothe reference ground; a seventeenth transistor having a first terminal,a second terminal and a control terminal, wherein the first terminal andthe control terminal are coupled to the second terminal of the thirdcontrollable current source; an eighteenth transistor having a firstterminal, a second terminal and a control terminal, wherein the firstterminal is coupled to the first terminal of the first current mirror,the second terminal is coupled to the frequency setting circuit, thecontrol terminal is coupled to the control terminal of the seventeenthtransistor; a nineteenth transistor having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the power supply voltage, the second terminal is coupled to thefrequency setting circuit, the control terminal is coupled to thecontrol terminal of the sixteenth transistor; and a sixth current sourcehaving a first terminal and a second terminal, wherein the firstterminal is coupled to the second terminals of the sixteenth transistorand the seventeenth transistor, the second terminal is coupled to thereference ground.
 11. A controller used in a switching converter,wherein the switching converter has a main transistor and is configuredto provide an output signal, the controller comprises: a clock generatorconfigured to generate a clock signal to determine the switchingfrequency of the main transistor; and a control circuit coupled to theclock generator, wherein based on the clock signal and a feedback signalindicative of the output signal of the switching converter, the controlcircuit generates a control signal to control the main transistor;wherein if the on-time of the main transistor is smaller than a timethreshold, the clock generator will adjust the frequency of the clocksignal to regulate the on-time of the main transistor to be equal to thetime threshold.
 12. The controller of claim 11, wherein the clockgenerator comprises: a first current control circuit having an inputterminal and an output terminal, wherein the input terminal is coupledto the control circuit to receive the control signal, and wherein basedon the control signal, the first current control circuit detects whetherthe on-time of the main transistor is smaller than the time threshold,and generates a first current control signal at the output terminal; afirst controllable current source having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto a power supply voltage, the control terminal is coupled to the outputterminal of the first current control circuit; a frequency settingcircuit configured to provide a setting current; a first current mirrorincluding a first transistor and a second transistor, wherein the firstcurrent mirror has a power supply terminal, a first terminal and asecond terminal, and wherein the power supply terminal is coupled to thepower supply voltage, the first terminal is coupled to the frequencysetting circuit and the second terminal of the first controllablecurrent source; a first capacitor having a first terminal and a secondterminal, wherein the first terminal is coupled to the second terminalof the first current mirror, the second terminal is coupled to areference ground; a fourth transistor having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the first terminal of the first capacitor, the second terminal iscoupled to the reference ground; a comparator having a first inputterminal, a second input terminal and an output terminal, wherein thefirst input terminal is coupled to the first terminal of the firstcapacitor, the second input terminal is configured to receive athreshold voltage; and a first one-shot circuit having an input terminaland an output terminal, wherein the input terminal is coupled to theoutput terminal of the comparator, the output terminal is coupled to thecontrol terminal of the fourth transistor and is configured to providethe clock signal.
 13. The controller of claim 12, wherein the frequencysetting circuit comprises: a third transistor having a first terminal, asecond terminal and a control terminal, wherein the first terminal iscoupled to the second terminal of the first controllable current sourceand the first terminal of the first current mirror; an operationalamplifier having a first input terminal, a second input terminal and anoutput terminal, wherein the first input terminal is configured toreceive a reference voltage, the second input terminal is coupled to thesecond terminal of the third transistor, the output terminal is coupledto the control terminal of the third transistor; and a first resistorhaving a first terminal and a second terminal, wherein the firstterminal is coupled to the second terminal of the third transistor, thesecond terminal is coupled to the reference ground.
 14. The controllerof claim 12, wherein the first current control circuit comprises: asecond one-shot circuit having an input terminal and an output terminal,wherein the input terminal is coupled to the control circuit to receivethe control signal; a flip-flop having a clock input terminal, a datainput terminal and an output terminal, wherein the clock input terminalis coupled to the output terminal of the second one-shot circuit, thedata input terminal is coupled to the control circuit to receive thecontrol signal, the output terminal is configured to provide an on-timedetection signal; a logic circuit having an input terminal, a firstoutput terminal and a second output terminal, wherein the input terminalis coupled to the output terminal of the flip-flop, and wherein based onthe on-time detection signal, the logic circuit generates a first logicsignal and a second logic signal respectively at the first outputterminal and the second output terminal; a first current source having afirst terminal and a second terminal, wherein the first terminal iscoupled to the power supply voltage; a fifth transistor having a firstterminal, a second terminal and a control terminal, wherein the firstterminal is coupled to the second terminal of the first current source,the control terminal is coupled to the first output terminal of thelogic circuit to receive the first logic signal; a sixth transistorhaving a first terminal, a second terminal and a control terminal,wherein the first terminal is coupled to the second terminal of thefifth transistor, the control terminal is coupled to the second outputterminal of the logic circuit to receive the second logic signal; asecond current source having a first terminal and a second terminal,wherein the first terminal is coupled to the second terminal of thesixth transistor, the second terminal is coupled to the referenceground; and a second capacitor having a first terminal and a secondterminal, wherein the first terminal is coupled to the second terminalof the fifth transistor and the first terminal of the sixth transistor,and is configured to provide the first current control signal, thesecond terminal is coupled to the reference ground.
 15. The controllerof claim 12, wherein the first controllable current source comprises: aseventh transistor having a first terminal, a second terminal and acontrol terminal, wherein the control terminal is coupled to the firstcurrent control circuit to receive the first current control signal; asecond resistor having a first terminal and a second terminal, whereinthe first terminal is coupled to the second terminal of the seventhtransistor; a third current source having a first terminal and a secondterminal, wherein the first terminal is coupled to the second terminalof the second resistor, the second terminal is coupled to the referenceground; and a second current mirror including an eighth transistor and aninth transistor, wherein the second current mirror has a power supplyterminal, a first terminal and a second terminal, wherein the powersupply terminal is coupled to the power supply voltage, the firstterminal is coupled to the first terminal of the seventh transistor, thesecond terminal is coupled to the first terminal of the first currentmirror.
 16. The controller of claim 14, wherein the first currentcontrol circuit further comprises: a fourth current source having afirst terminal and a second terminal, wherein the first terminal iscoupled to the power supply voltage; a tenth transistor having a firstterminal, a second terminal and a control terminal, wherein the firstterminal is coupled to the power supply voltage, the second terminal iscoupled to the first terminal of the second capacitor, the controlterminal is coupled to the second terminal of the fourth current source;an eleventh transistor having a first terminal, a second terminal and acontrol terminal, wherein the first terminal is coupled to the controlterminal of the tenth transistor, the second terminal is coupled to thereference ground, the control terminal is coupled to the first terminalof the second capacitor; a multi detection circuit coupled to the outputterminal of the flip-flop, wherein based on the on-time detectionsignal, the multi detection circuit determines whether the on-time ofthe main transistor is smaller than the time threshold in a plurality ofsuccessive switching cycles, and generates a multi detection signal; anda twelfth transistor having a first terminal, a second terminal and acontrol terminal, wherein the first terminal is coupled to the controlterminal of the tenth transistor, the second terminal is coupled to thereference ground, and the control terminal is coupled to the multidetection circuit to receive the multi detection signal.
 17. Thecontroller of claim 12, wherein the first current control circuitcomprises: a thirteenth transistor having a first terminal, a secondterminal and a control terminal, wherein the first terminal is coupledto the power supply voltage, the control terminal is coupled to thecontrol terminals of the first transistor and the second transistor; afourteenth transistor having a first terminal, a second terminal and acontrol terminal, wherein the first terminal is coupled to the secondterminal of the thirteenth transistor; a fifteenth transistor having afirst terminal, a second terminal and a control terminal, wherein thefirst terminal is coupled to the second terminal of the fourteenthtransistor, the control terminal is coupled to the control circuit toreceive the control signal; a fifth current source having a firstterminal and a second terminal, wherein the first terminal is coupled tothe second terminal of the fifteenth transistor, the second terminal iscoupled to the reference ground; and a third capacitor having a firstterminal and a second terminal, wherein the first terminal is coupled tothe second terminal of the fourteenth transistor and the first terminalof the fifteenth transistor, and is configured to provide the firstcurrent control signal, the second terminal is coupled to the referenceground.
 18. The controller of claim 12, wherein the clock generatorfurther comprises: a second current control circuit configured to detectwhether the switching converter works under light load condition andgenerate a second current control signal; and a second controllablecurrent source having a first terminal, a second terminal and a controlterminal, wherein the first terminal is coupled to the power supplyvoltage, the second terminal is coupled to the first terminal of thefirst current mirror, the control terminal is coupled to the secondcurrent control circuit to receive the second current control signal.19. A control method of a switching converter, wherein the switchingconverter has a main transistor and is configured to provide an outputsignal, the control method comprises: generating a feedback signalindicative of the output signal of the switching converter; generating aclock signal to determine the switching frequency of the maintransistor; generating a control signal to control the main transistorbased on the clock signal and the feedback signal; detecting whether theon-time of the main transistor is smaller than a time threshold based onthe control signal; and if the on-time of the main transistor is smallerthan the time threshold, adjusting the frequency of the clock signal toregulate the on-time of the main transistor to be equal to the timethreshold.
 20. The control method of claim 19, further comprising:detecting whether the switching converter works under a light loadcondition; and decreasing the frequency of the clock signal if theswitching converter works under the light load condition.